The present disclosure generally relates to semiconductor device manufacture, and, more particularly, to methods for fabricating optically-pumped vertical cavity surface-emitting lasers (xe2x80x9cVCSELsxe2x80x9d).
Optical communication systems are now commonly used for exchanging information via light wave signals. Laser diodes or xe2x80x9csemiconductor lasersxe2x80x9d are often the preferred optical source for such an optical communication system. This is due, primarily, to the narrow xe2x80x9cspectral linewidthxe2x80x9d (or range of wavelengths) at which these devices produce light and their fast modulation rates.
Semiconductor lasers generally include an xe2x80x9coptical cavityxe2x80x9d with an xe2x80x9cactive regionxe2x80x9d arranged between two xe2x80x9creflectors.xe2x80x9d Light from the active region bounces back and forth between these reflectors, gaining intensity with each cycle, until a portion of the light is allowed to escape the oscillator cavity. So-called xe2x80x9cedge-emittingxe2x80x9d lasers produce light from between the layers of the structure, substantially parallel to the active region, while xe2x80x9csurface-emittingxe2x80x9d lasers emit light substantially perpendicular to the layers forming the active region.
xe2x80x9cVertical-cavityxe2x80x9d surface-emitting lasers, or VCSELs, have a number of advantages over traditional edge-emitting lasers, including low manufacturing cost, good beam quality, low current operation and scalability. These properties make VCSELs desirable for many applications, and VCSELs that can produce long-wavelength light (1300 nm-1550 nm) are of particular interest in optical communications. In VCSELs, the optical cavity is typically arranged between two xe2x80x9cdistributed Bragg reflectors,xe2x80x9d or xe2x80x9cDBRs.xe2x80x9d In simple terms, each of these DBRs consists of alternating layers of materials where each layer is partially reflective and has a different xe2x80x9crefractive indexxe2x80x9d and thickness. The reflections from each of the layers add together in unison depending upon the wavelength of light. Consequently, DBRs act essentially as wavelength-selective, or filtering, reflectors for returning light over a range of wavelengths. The range of wavelength or xe2x80x9cstopbandxe2x80x9d depend on the index contrast and layer thicknesses.
VCSELs may be electrically or optically driven. A conventional xe2x80x9ccurrent-injectionxe2x80x9d VCSEL includes two ohmic contacts for applying an electrical current to the active region. Typically, one of the ohmic contacts is located below the substrate, while the other ohmic contact is located above the top DBR. When voltage is applied to the contacts, electrical current is injected into the active region, causing the active region to emit light. Instead of electrical current, a conventional xe2x80x9coptically pumpedxe2x80x9d VCSEL includes, or is operationally associated with, a light source. For example, the pump may be another VCSEL or a light-emitting diode. Light from the pump enters the active region and produces the stimulated emission.
U.S. Pat. No. 5,513,204 to Jayaraman describes an optically pumped VCSEL device that includes a short-wavelength VCSEL that optically pumps a long-wavelength VCSEL. The long-wavelength VCSEL is coupled to the short-wavelength VCSEL by a layer of adhesive material. The adhesive material may be a transparent, optical adhesive material or a metallic bonding material. In an alternative embodiment, the VCSELs are fusion bonded to form a monolithic structure.
Another optically pumped VCSEL device is described in U.S. Pat. No. 5,754,578, which is also issued to Jayaraman. One embodiment of this VCSEL device includes a short-wavelength VCSEL and a long-wavelength VCSEL that are formed on a single GaAs substrate. The short-wavelength VCSEL includes a current confining scheme which may be provided by proton implantation or by an oxidation layer. U.S. Pat. Nos. 5,513,204 and 5,754,578 are incorporated by reference into this application.
Of particular interest, the Jayaraman devices use wafer bonding, epoxy bonding or metal-to-metal bonding to form VCSEL devices. Such bonding techniques are known to exhibit voids and non-uniform bonding of the wafers of the VCSEL devices. These non-uniform conditions can cause mechanical stress in the devices and can lead to poor device performance.
What is needed is a laser that exhibits good beam quality, low current operation and scalability. Such a laser also should be fabricated using techniques that reduce mechanical stress so that the performance of the laser does not become degraded during operation.
The invention involves the fabrication of optically-pumped lasers through the use of flip-chip bonding techniques. Flip-chip bonding can help reduce or eliminate non-uniformities due to voids and poor bonding across the wafers of the optically-pumped lasers. Flip-chip bonding also can reduce the high stress conditions typically associated with wafer-bonded, unmatched material systems that can lead to poor reliability. These techniques also can enable an optically-pumped long-wavelength VCSEL of an optically-pumped laser device to be heatsunk through the short-wavelength VCSEL down to a heat-dissipating structure.
A method for fabricating an optically-pumped laser includes filling a well of one of the VCSELs with solder paste. The solder paste is then heated to a temperature above a wetting-point temperature of the solder paste, and then cooled to a temperature below the wetting-point temperature to form a solder bump. After cooling, the solder bump protrudes above the top of the well. To attach the VCSELs to each other, the solder bump can be engaged with the other VCSEL. Then, the solder bump can be reheated to a temperature above the wetting-point temperature and subsequently cooled to a temperature below the wetting-point temperature. This process affixes the VCSELs to each other. Note, in some embodiments, the solder bump can be reheated until the pad of the other of the VCSELs aligns with the solder bump.
Various techniques can be used to fill the well of a VCSEL with solder. For instance, the solder can be applied by vaporization, electroplating, printing, sputtering, setting, stud-bumping or direct placement.
Flip-chip bonding enables the short- and long-wavelength VCSEL devices to be processed and optimized separately. The devices can then be attached on a wafer-scale or as discrete devices. Wafer-scale attachment enables many devices to be attached at the same time, resulting in cost savings due to reduced assembly time. However, discrete device flip-chip bonding also can be done at the end of the assembly process, thus providing flexibility in wavelength selection, since the wavelength of the product can be determined near the end of the assembly process.
An embodiment of an optical communication system of the invention includes an optically-pumped laser, an optical waveguide and an optical detector. In particular, the optically-pumped laser includes a short-wavelength VCSEL pump that is flip-chip bonded to a long-wavelength VCSEL. The optical waveguide optically communicates with the optically-pumped laser and is used to transmit light from the optically-pumped laser. The optical detector optically communicates with the optical waveguide and is used to detect light from the waveguide.
Clearly, some embodiments of the invention may address shortcomings of the prior art in addition to, or in lieu of, those described here. Additionally, other systems, methods, features, and advantages of the present invention will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.